1. Field of the Invention
The invention relates to a paving asphalt composition or asphalt binder utilized in paving asphalt road surfaces which provides increased durability, and thus longevity, in a wide range of temperatures. Preferably, the paving asphalt composition is formed using asphalt base stock, heavy residuum of re-refined motor oil, and a chemical modifier. The paving asphalt composition surprisingly provides increased flexibility in extremely cold temperatures, and increased stiffness in extremely hot temperatures thereby increasing the durability of the asphalt road surface over a wide range of temperatures.
2. Description of Related Art
Fluctuating temperatures have an adverse effect on asphalt road surfaces. "Asphalt road surfaces" are defined herein as road surfaces which have been paved with asphalt-based concrete mixtures such as hot mix asphalt concrete. Should the temperature of the asphalt road surface become too hot, the modulus or stiffness of the asphalt decreases and rutting occurs. Rutting is caused by vehicles traveling over a road surface which is not stiff and/or elastic enough to carry the weight of the vehicles. The asphalt road surface is deformed such as by ruts or grooves forming in the asphalt road surface.
Should the temperature become too cold, the flexibility of the asphalt road surface decreases thereby causing the asphalt road surface to become brittle resulting in cracking of the asphalt road surface. Cracking is caused by vehicles traveling over a road surface which does not provide sufficient flexibility to support the weight of the vehicles. Cracking may also occur due to thermal cracking caused when the asphalt road surface shrinks as the temperature drops until stresses in the asphalt road surface increase to the point that they exceed the critical tensile strength causing the asphalt road surface to crack. Accordingly, it is an object of the present invention to provide an paving asphalt composition, or asphalt binder, which provides an asphalt road surface which maintains its flexibility in cold weather and its stiffness in hot weather.
Generally, paving asphalt compositions may include a hard, low penetration asphalt base stock and an oil or cutter stock such as vacuum gas oil. The asphalt base stock is the component which prevents rutting at high temperatures. The oil is the component which prevents cracking at low temperatures. The addition of the oil to the paving asphalt compositions also reduces the affect the asphalt base stock has on preventing rutting and permanent deformation. Prior paving asphalt compositions have encountered difficulties in maintaining a dynamic shear greater than about 1 kPa at a high temperature (measured by the formula G*/sin.delta. of AASHTO TP5, American Association of State Highway and Transportation Officials), while simultaneously maintaining a direct tension greater than 1% strain at a low temperature (measured by AASHTO TP3, American Association of State Highway and Transportation Officials). Most notably, prior asphalt compositions which provide increased flexibility, and thus increased durability in cold climates, do not provide increased stiffness, and thus increased durability in hot climates. Likewise, prior asphalt compositions which provide increased durability in hot climates do not provide durability in cold climates.
Prior paving asphalt compositions have also included rubber and/or other polymers in an attempt to improve the direct tension at low temperatures and/or the viscoelastic properties, such as dynamic shear, at high temperatures. While such paving asphalt compositions have demonstrated a significant improvement over paving asphalt compositions which do not include rubber and/or polymers, the use of rubber and other polymers is extremely expensive. Accordingly, it is an object of the present invention to develop a paving asphalt composition which simultaneously has a dynamic shear greater than about 1 kPa at a surprisingly high temperature, and a direct tension greater than about 1% at a surprisingly low temperature, without the need for using as much, if any, expensive rubber and other polymers compared to prior paving asphalt compositions.
The susceptibility of the prior paving asphalt compositions to varying temperatures make these paving asphalt compositions poor choices for use in asphalt road surfaces where, during the course of a year, the temperature fluctuates over a wide temperature range. Accordingly, it is an object of the present invention to develop a paving asphalt composition which simultaneously provides increased durability in hot climates and cold climates thereby reducing costs of production relative to prior paving asphalt compositions. In a preferred aspect, as discussed below, a specific version of the invention yields a paving asphalt composition which provides surprisingly effective results in maintaining the flexibility of the asphalt road surface in cold temperatures and the stiffness of the asphalt road surface in hot temperatures.